Real-time eye motion correction in phase-resolved OCT angiography with tracking SLO

Abstract

In phase-resolved OCT angiography blood flow is detected from phase changes in between A-scans that are obtained from the same location. In ophthalmology, this technique is vulnerable to eye motion. We address this problem by combining inter-B-scan phase-resolved OCT angiography with real-time eye tracking. A tracking scanning laser ophthalmoscope (TSLO) at 840 nm provided eye tracking functionality and was combined with a phase-stabilized optical frequency domain imaging (OFDI) system at 1040 nm. Real-time eye tracking corrected eye drift and prevented discontinuity artifacts from (micro)saccadic eye motion in OCT angiograms. This improved the OCT spot stability on the retina and consequently reduced the phase-noise, thereby enabling the detection of slower blood flows by extending the inter-B-scan time interval. In addition, eye tracking enabled the easy compounding of multiple data sets from the fovea of a healthy volunteer to create high-quality eye motion artifact-free angiograms. High-quality images are presented of two distinct layers of vasculature in the retina and the dense vasculature of the choroid. Additionally we present, for the first time, a phase-resolved OCT angiogram of the mesh-like network of the choriocapillaris containing typical pore openings.

Keywords: (110.0110) Imaging systems; (110.4500) Optical coherence tomography; (170.3880) Medical and biological imaging; (170.4470) Ophthalmology; (280.2490) Flow diagnostics.

Fig. 1

The optical layout of the combined OFDI-TSLO setup. (TSLO PANEL) The TSLO schematic with its optical paths in blue. A super luminescent diode (SLD) was used as the light-source combined with an acoustic-optic modulator (AOM). The light is directed via a set of lenses to three curved mirror telescopes to image a horizontal scanner (HS) and a vertical scanner (VS) onto the pupil of the eye. A beam splitter (BS) was used to relay returning light to a photo-multiplier tube (PMT) for detection. A pinhole (P) is used to provide confocal retinal imaging and two diaphragms (D) are used to control the input beam size on the eye and to block light scattered within the setup from the PMT. (OFDI PANEL) The OFDI schematic with its optical paths in red. Light from a wavelength swept-source is passed on into a fiber-based interferometer that splits into a sample arm to the eye and a reference arm. Scattered light from the eye is recombined with the reference arm light at a 50/50 coupler and detected with balanced detection (BD). An additional Mach-Zehnder interferometer is attached to the reference arm with a 90/10 coupler to simultaneously measure a calibration signal and forms the second interferometer channel. Interferometric detection was optimized using in-line polarization controllers (PC). (OFDI-TSLO PANEL) The TSLO and the OFDI were coupled together via a dichroic mirror (DM).

Fig. 2

In vivo phase-noise measurement due to sample motion. (A) The OCT galvanometer scanners were stopped and A-scans were acquired for a period of 0.1s (10,000 A-scans). The top figure shows the intensity image, the bottom figure shows the phase-difference image for a time interval of 1.0 ms (100 A-scans). The phase-noise was analyzed from the RPE layer (yellow box) to avoid interference with blood flow. (B) The spot displacement due to lateral sample motion without eye tracking in blue solid dots and with eye tracking in red open dots. To improve the visibility of individual data points the results with eye tracking are slightly shifted to the right.

Fig. 3

Examples of eye motion artifacts in phase-resolved OCT angiography of the retina and its correction by eye tracking. The horizontal and vertical eye position are plotted below the angiograms respectively in blue and red. The angiograms display a surface area of 2.0 x 2.0 mm². The green scale-bars are 250 µm in length. (A) Untracked. The angiogram is corrupted by microsaccades which create discontinuity artifacts in the visualized vasculature (examples are highlighted by yellow boxes) and white line artifacts. (B) Tracked. The angiogram still shows the line artifacts but the discontinuity artifacts are absent (examples are highlighted by yellow boxes). (C) Tracked + Invalid B-scans rescanned. The TSLO is able to detect untracked (micro)saccades (marked by * in the eye position graph) and signaled the OFDI to reacquire the B-scans during these events. This angiogram includes the corrupted B-scans. (D) Tracked + Invalid B-scans removed. In post-processing the corrupted B-scans were removed. A single undetected artifact remained at the right side.

Fig. 4

(A) The segmentation of layers with different vascular networks in an OCT intensity B-scan. The first retinal layer (red - green) consists of the nerve fiber layer (NFL), the ganglion cell layer (GCL) and the inner plexiform layer (IPL). The second retinal layer is a 73 µm thick layer below the IPL (green - blue) that consists of the inner nuclear layer (INL) and the outer plexiform layer (OPL). The choriocapillaris is segmented as a 18 µm thick layer below the RPE (yellow - magenta) and the choroid is segmented as a 90 µm thick layer below the choriocapillaris (magenta - orange). (B) The segmentation boundaries overlayed on a flow B-scan. Blood flow is shown as white signals against a black background. (C) Angiogram of the first retinal layer NFL-GCL-IPL. (D) Angiogram of the second retinal layer INL-OPL. (E) Angiogram of the choriocapillaris. (F) Angiogram of the choroid. The angiograms display a surface area of 2.0 x 2.0 mm². The yellow scale-bars are 250 µm in length.

Fig. 5

High-quality artifact-free angiograms of the retina by compounding eight data sets from the same location with a surface area of 2.0 x 2.0 mm². The yellow scale-bars are 250 µm in length. (A) Angiogram of the NFL-GCL-IPL layer clearly showing the vasculature around the foveal avascular zone. (B) Angiogram of the INL-OPL layer showing a fine vascular network. (C) Three-dimensional rendering of the entire retinal vasculature to visualize its orientation in depth. The three-dimensional rendering is also available in a movie: high-resolution (Media 1) and low-resolution (Media 2).

Fig. 6

High-quality artifact-free angiograms of the choroid and the choriocapillaris by compounding eight data sets from the same location. The black scale-bars are 250 µm in length, the white scale-bars are 40 µm in length. (A) In vivo angiogram of the choroid showing a dense network of large vessels below the choriocapillaris for a 2.0 x 2.0 mm² area. (B) In vivo angiogram of the choriocapillaris showing a mesh-like network of small vessels with small black pores for a 2.0 x 2.0 mm² area. (C) A magnified view on a 410 x 410 µm² area of the choriocapillaris angiogram (marked yellow in (B)) which shows individual pores in detail from which five examples are indicated with white arrows. (D) Ex vivo scanning electron microscopy image from a methyl methacrylate cast of an excised eye which shows the subfoveal choriocapillaris microstructures in high-resolution for a 1.8 x 1.5 mm² area. The choriocapillaris structures are marked by an orange box; remnants of the retinal vasculature are seen outside the box. The choriocapillaris angiogram of (B) greatly resembles the microstructures shown in this image. The image was reproduced and adapted from Olver et al. [41] with the permission of the Nature Publishing Group. (E) A magnified view from (D) for a 410 x 410 µm² area (marked red in (D)) in which white arrows indicate pores with a similar size as those observed in (C).